1. Field of the Invention
The present invention relates to methods of preparing pigments which can be used in compositions including inkjet inks.
2. Description of the Related Art
In general, pigments alone are not readily dispersible in liquid vehicles. A variety of techniques have been developed which can provide stable pigment dispersions. For example, dispersants can be added to the pigment to improve its dispersibility in a particular medium. Examples of dispersants for an aqueous medium include water-soluble polymers and surfactants.
The surface of pigments contain a variety of different functional groups, and the types of groups present depend on the specific class of pigment. Methods have been developed in which materials, in particular, polymers, are grafted to a pigment utilizing the functional groups present on the surface. For example, it has been shown that polymers can be attached to carbon blacks containing surface groups such as phenols and carboxyl groups. However, methods which rely on the inherent functionality of a pigment's surface cannot be applied generally because not all pigments have the same specific functional groups.
Furthermore, several methods have been developed for modifying the types or amounts of surface functional groups of pigments in order to improve performance, including dispersibility. For example, pigments, such as carbon black, have been reacted with oxidants including ozone and nitric acid. The resulting pigments have been found to have a higher degree of oxygen-containing groups on the surface, which has been shown to improve dispersibility in aqueous medium such as water. Other surface modification methods, such as chlorination and sulfonylation are also known.
A method of preparing modified colored pigments have also been developed which provide ink compositions with improved properties, such as dispersibility. For example, U.S. Pat. No. 5,851,280 discloses methods for the attachment of organic groups onto pigments including, for example, attachment via a diazonium reaction wherein the organic group is part of the diazonium salt. The resulting surface-modified pigments can be used in a variety of applications, such as inks, inkjet inks, coatings, toners, plastics, rubbers, and the like.
PCT International Publication No. WO 01/51566 describes a method of making a modified pigment by reacting a first chemical group and a second chemical group to form a pigment having attached a third chemical group. The first chemical group includes at least one nucleophile and the second chemical group includes at least one electrophile, or vice versa. These pigments are used in ink compositions and, in particular, inkjet ink compositions.
While these methods provide modified pigments having attached groups, there remains a need for improved processes for preparing pigments which have improved properties, including dispersibility. These additional methods may provide advantageous alternatives to forming modified pigments and may produce compositions, such as inkjet inks, with improved overall performance.